Our Chardonnay clusters are getting larger and berries are starting to touch each other. It feels a bit early, but here are grape disease management reminders at berry touch/cluster closure stage.
Botrytis gray mold: The best management practice is canopy management. Botrytis pathogen likes high humidity, thus, the poorly managed canopy that traps humidity will help them to thrive. This pathogen also needs wounds to develop spores, thus, grape berry moth and other insects plus birds need to be managed too.
There is a number of Botrytis materials such as Rovral and Meteor (FRAC 2), Elevate (FRAC 17), Vanguard and Scala (FRAC 9), Luna Experience (FRAC 7 plus 3), Kenja (FRAC 7), Miravis Prime (FRAC 7 plus 12), Switch (FRAC 9 plus 12), etc. Botrytis pathogen is well known for its ability to develop fungicide resistance, so, please rotate FRAC codes!!
QoI fungicides (FRAC 11) are no longer the effective material for us due to the development of QoI-resistant Botrytis isolates throughout Virginia. Pristine (FRAC 11 plus 7) has been compromised as well. The next timing of application for Botrytis management is at veraison.
Ripe rot and bitter rot: At this point of the season, captan (M4) and QoI (Strobirulin, FRAC 11) fungicides are good options. In addition, we found that Switch (9 plus 12), copper (M1), tebuconazole (3), Aprovia (7), and Ph-D (19) are somewhat effective. However, none of these materials consistently provided satisfactory suppression of ripe rot when we applied these materials by itself in a series of field studies. Thus, please mix two FRAC codes, especially if you have ripe rot issues in the past. The timing of applications will be the same as Botrytis.
Sour rot: A tank mix of the insecticide zeta-cypermethrin (Mustang Maxx) and the antimicrobial hydrogen dioxide (OxiDate 2.0) before a symptom appears (before veraison) to suppress the fruit fly population is the key. In NY, there are reports of Mustang Maxx-resistant fruit flies, thus, please do not overuse it. Two applications should be sufficient. If you don’t have OxiDate, Switch also lists sour rot (suppression only), and other broadspectrum fungicides such as captan and copper, probably have some efficacy too.
Speaking of sour rot, please take a short survey below. We try to gain more information from growers so that we can come up with research priorities. It will be closed in 5 days, so, please participate today. It is not only for Virginia. Thus, if you grow grapes in other states or country, we need your opinion too!